1. Field of the Invention
The present invention relates to a display device for displaying a picture, and more particularly, to a display device utilizing a piezoelectric material and a method for fabricating the same.
2. Discussion of the Related Art
A liquid crystal display (LCD) device, a flat panel display devices, has drawn attention as a substitute for a cathode ray tube. The LCD is a device in which an electric field is applied to liquid crystal, which has fluidity of liquid and optical properties of crystal, in order to vary optical anisotropy of the liquid crystal. The LCD has been widely used as a display device because it consumes less power, is compact, and can be produced with a relatively large screen. In addition, the LCD can realize a high definition picture.
The LCD device is provided with an upper substrate (often referred to as a color filter substrate), and a lower substrate (often referred to as a thin film transistor substrate), both of which are arranged opposite to each other. Liquid crystal having a dielectric anisotropy is disposed between the first and second substrates for displaying a picture by utilizing transmittivity of light varied by changing orientation of the liquid crystal.
A twisted nematic (TN) type of liquid crystal is used mostly in fabricating an LCD device. The TN type requires an alignment film to control the initial orientation of the liquid crystal. Alignment films are formed on the inside surfaces of each of the upper and lower substrates. Their alignment directions are determined by rubbing in different directions.
On the outside surfaces of the two substrates, there are polarizing plates, of which the transmission axes are set to be either a normally black mode (hereafter referred to as an NB mode) or a normally white mode (hereafter referred to as an NW mode). For an example, a 90° TN LCD is fabricated by rubbing the alignment films of the upper and lower substrates with a 90° difference and filling nematic liquid crystal mixed with cholesteric liquid crystal to form a liquid crystal layer having a thickness of 4˜8 μm and a 60˜100 μm pitch between the lower and upper substrates. Thereafter, a mode of operation (either an NB mode or an NW mode) is determined by tweaking the axes of transmittivity of the polarizing plates attached to the outside surfaces of the upper and lower substrates.
In the NB mode, screen is dark if no voltage is applied thereto, and screen is bright if a voltage higher than the threshold voltage is applied thereto. Conversely, in the NW mode, screen is bright if no voltage is applied thereto, and screen is dark if a voltage higher than the threshold voltage is applied thereto. A picture can be displayed in LCD devices by using such a principle.
FIG. 1 illustrates a cross-section of the related art liquid crystal display device, FIG. 2 illustrates a plan view of the related art liquid crystal display device, and FIGS. 3A and 3B illustrate perspective views each showing the driving principle of the related art liquid crystal display device.
Referring to FIG. 1, the related art liquid crystal display device is provided with a first substrate 1 having a color filter layer 12 formed thereon for producing color, a second substrate 2 having switching devices (i.e., thin film transistors (TFTs) for shifting orientation of liquid crystal molecules, and a liquid crystal layer between the first and second substrates 1, and 2.
On the first substrate 1, there are a color filter layer 12 at each of pixel regions for producing color, a black matrix layer 11 at portions except for the pixel regions to shield light, and a common electrode 13 to apply a voltage to the liquid crystal layer 3.
Referring to FIG. 2, on the second substrate 2, there are a plurality of gate lines 32 and data lines 35 perpendicular to one another to define sub-pixel regions. A plurality of pixel electrodes 22 that are electrically connected to each TFT are formed to apply a voltage to the liquid crystal layer at the pixel regions. Also, there are a plurality of thin film transistors TFT at cross portions of the gate lines 32 and the data lines 35.
In addition, the second substrate 2 has a storage capacitor electrode 36 which is overlapped the pixel electrode 22 for sustaining a voltage charged to the liquid crystal layer 3 during the turn-off period of the thin film transistor.
Between the first and second substrates 1, and 2, spacers 15 maintain a gap between the two substrates. At the edges of the substrates, there is sealant 16 to prevent the liquid crystal from flowing out, and to attach the first and second substrates 1 and 2 together.
In the foregoing liquid crystal display panel, a desired molecular arrangement can not be obtained by placing the liquid crystal only between the two substrates 1 and 2. Thus, it is essential that the first and second alignment films 51 and 52 be formed on the inside walls of the substrates in contact with the liquid crystal.
In a method for forming the alignment film, an organic polymer thin film is formed on the substrate by the processes of printing and hardened and the film is rubbed to control the orientation of the liquid crystal molecules. Alternatively, there is a photo-orientation method which can be performed without physical contact, such as rubbing. Also, a static electricity can be avoided in the photo-orientation method if a photo-sensitive alignment film is used. Thus, the initial orientation of the liquid crystals is determined by rubbing the alignment film in a direction, or directing a polarized ultraviolet beam.
First and second polarizing plates 61, and 62 are attached to the outside surfaces of the first and second substrates 1 and 2 such that polarization axes thereof are perpendicular to each other. The polarizing plates are essential and important components for providing transmissive light in a predetermined direction. More specifically, by rubbing the first and second alignment films on the inside surfaces of the first and second substrates 1 and 2 perpendicular to each other, the liquid crystal molecules are initially twisted. In this instance, the arrangement of the liquid crystal molecules can be twisted because the liquid crystal close to the surface of the alignment film is orientated in a direction the same as the rubbing direction of the alignment film, so that a vertical alignment is performed.
Referring to FIG. 3A, the transmissive axes of the first and second polarizing plates 61, and 62 attached to the outside surfaces of the first and second substrates 1 and 2 are also perpendicular to each other. The directions of the transmissive axes of the first and second polarizing plates 61, and 62 and the directions of rubbing of the first and second alignment films are disposed in parallel, respectively. Therefore, the light incident from a vertical direction on the second substrate 2 is polarized at the second polarizing plate 62, turned by 90° following the twist of the liquid crystal molecules 3 while the light passes through the liquid crystal, and passes through the first polarizing plate 61 to display a picture.
Referring to FIG. 3B, if a voltage higher than a predetermined value is applied to the device, an electric field is formed between the common electrode and the pixel electrode, to re-orient the liquid crystal molecules parallel to the direction of the electric field. Accordingly, because the light incident from the vertical direction on the second polarizing plate 62 is polarized, passes a liquid crystal cell in parallel and fails to pass through the first polarizing plate 61. As a result, the light fails to display a picture. Thus, the liquid crystal serve as a shutter that cuts off light by applying a voltage thereto.
That is, if no voltage is applied to the device, the liquid crystal 3 are arranged as shown in FIG. 3A, to display the white state. If a voltage higher than a threshold value is applied to the device, the liquid crystal 3 is arranged parallel to the electric field as shown in FIG. 3B, to display the black state. By using this principle, black can be displayed on the white base. Conversely, white can be displayed on the black base.
However, the related art liquid crystal display device has the following problems. First, in fabricating a display device by applying the liquid crystal thereto, the alignment films are required for initial orientation of the liquid crystal. However, the alignment film can be contaminated such that their performance is affected. Also, alignment performance can be deteriorated by process steps before or after forming the alignment film, thereby resulting in decreasing yield. Second, due to the nature of fabrication process, such as the steps of forming various patterns on two substrates opposite to each other and forming a liquid crystal layer therebetween, there is a high probability in manufacturing failure in each manufacturing step. Third, there are many problems in view angle, response speed, residual image, and light leakage that can be caused by inconsistent viscosity and initial orientation of the liquid crystal molecules, thereby deteriorating the picture quality of the display device.